The invention relates generally to an information recording disk, and more particularly to a recording disk which can be used as a magneto-optical recording medium.
The specification for conventional information recording disks are detailed in ISO/TC97/SC23 Document No. 126, item 4.9. As shown in FIG. 10 an information recording disk 101 constructed according to this publication includes a hub 104 mounted on a first substrate 11 over a center aperture 102 of disk 101. Alternatively, a hub can be mounted on both a second substrate 12 and first substrate 11. In either embodiment, the hub is bonded to the surface of a substrate which includes an information recording layer by an adhesive or ultrasonic welding.
Conventional methods for mounting a hub on an information recording disk substrate such as bonding or ultrasonic welding adversely affect the quality of the information recording disk by increasing the birefringence of the substrate. Data in FIG. 12 illustrates how the birefringence of the substrate is affected by welding a hub to the substrate in a conventional arrangement. The double path variation in birefringence was measured using a 633 nm He--Ne parallel beam laser After a hub was ultrasonically welded to a polycarbonate substrate, the birefringence of the substrate increased by about 40 nm at a radius of about 23 mm. Such an increase in birefringence can adversely affect the quality of an information recording disk such as by degrading the signal qualities and leading to unstable operation of servomechanisms.
Conventional arrangements for mounting hubs onto recording disk substrates have other shortcomings as shown in FIG. 11. A disk 110 having a hub 111 mounted over a center aperture 114 is mounted on a spindle 112. When disk 110 which hub 111 is mounted in a drive, it is necessary that disk 110 be kept away from contacting spindle 112 and other guide members. Accordingly, it is necessary that components be designed so that the height of hub 111 does not raise such a problem. A conventional hub typically has a thickness of between about 2 to 3 mm. This does not usually present a problem when the drive is designed to accommodate such dimensions However, when it is required to reduce the thickness of the drive and the disk, the thickness of the hub becomes a serious factor. Accordingly, if two disk substrates each having a thickness of 1.2 mm are bonded together this presents a total thickness of 2.4 to 2.5 mm, and if a hub having a thickness of 2.2 mm is bonded to the substrates, the overall thickness is 4.6 to 4.7 mm, which is considered to be too large.
Additional problems are presented by the conventional hub mounting arrangements of FIG. 11 during operation. When disk 101 is placed in a drive and is rotated by spindle 112 of a drive motor, considerable heat from the drive motor is transmitted through hub 111 to substrate 115. As a result, the disk becomes distorted by the heat leading to changes in optical birefringence and mechanical properties. The extent to which various portions of the disk are influenced by the heat depends on the distance from the hub. The closer the arrangements, the greater the influence. Therefore, if the diameter of hub 111 is large the outer edge of hub 111 will be in close proximity to a recording layer 113 of disk 110 and the influence cannot be overlooked. Accordingly, this hub mounting arrangement detracts from the quality of the information recording disk by heating information recording layer 113 unevenly.
Accordingly, it is desireable to provide an information recording disk having an improved hub mounting arrangement which allows the disk to be thinner than conventional recording disks and does not suffer from other shortcomings of the prior art.